Keep in mind just how many wires lead to your tank--the heater, power heads, pump for skimmer, etc. Any one of them can be the source of the current. I'd have only one plugged in at a time until you find the cause. Make sure to test them all though.The multimeter test will help you determine whether it's really the pump though. Start at the max AC range, then adjust to lower ranges until the digital reading or analog needle starts to give a significant reading. The red probe will be in the water, and the black to a good ground.